The pieces suddenly fell into place. "Actually, I'm not drawing or writing. I'm counting windows," Peregrine explained, as if this were a perfectly ordinary thing to do. He showed the Ravenclaw the tally marks in his book. "I've counted one-hundred and fifty-eight so far. Well - I thought I had. This corridor has one more window on it than the floor below. Or I've miscounted. I suppose I'll have to count them a few times, anyway, to be sure my results are accurate."